1. Field of the Invention
The present invention relates to a conductive endless belt (hereinafter simply referred to as “belt” in some cases) and an image forming apparatus using the same, the endless belt being used when a toner image is transferred to a recording medium such as paper in an electrostatic recording process performed in an electrostatic recording apparatus or an electrophotographic apparatus such as a copy machine or a printer, the toner image being formed by supplying a developer onto the surface of an image forming member such as a latent image holding member holding a latent image thereon.
2. Description of the Related Art
Heretofore, in an electrostatic recording process performed in a copy machine, a printer, or the like, printing is performed by the steps of uniformly electrifying the surface of a photosensitive member (latent image holding member), forming an electrostatic latent image by projecting an optical image from an optical system onto this photosensitive member to diselectrify the area to which light is applied, then supplying toner to this electrostatic latent image to form a toner image by electrostatic adhesion of the toner, and transferring the toner image to a recording medium such as paper, transparent paper for overhead projector use, or photographic paper.
Also in a color printer or color copy machine, although the printing is fundamentally performed in accordance with the process described above, in the case of color printing in which four color toners, magenta, yellow, cyan, and black, are used for reproducing a color tone, a step of overlapping the color toners at a predetermined ratio is required, and in order to execute this step, various methods have been proposed.
As a first method, an image on image development method may be mentioned in which, as is the case of monochromatic printing, in order to form an electrostatic latent image into a visible color toner image, the above four color toners, magenta, yellow, cyan, and black, are sequentially supplied onto a photosensitive member so as to be superimposed for development. By the method described above, a relatively compact apparatus can be formed; however, it is very difficult to control the gradation, and as a result, a problem may arise in that a high quality image is not obtained.
As a second method, a tandem method using four photosensitive drums may be mentioned in which after latent images on the drums are developed by respective color toners, magenta, yellow, cyan, and black to form four toner images of magenta, yellow, cyan, and black, the photosensitive drums provided with the above respective toner images thereon are aligned, and the toner images thereof are then sequentially transferred to a recording medium such as paper for superimposing the images thereon, thereby reproducing a color image. By this method, superior image can be obtained; however, since the four drums each provided with an electrification mechanism and a development mechanism are aligned, the apparatus becomes large and expensive.
In FIG. 2, one example of a printing portion of a tandem image forming apparatus is shown. Four printing units each composed of a photosensitive drum 1, an electrification roller 2, a developing roller 3, a developing blade 4, a toner supply roller 5, and a cleaning blade 6 are provided for respective yellow Y, magenta M, cyan C, and black B toners, and the toners are sequentially transferred onto paper transported by a transfer/transport belt 10 which is circularly driven by a drive roller (drive member) 9, thereby forming a color image. Electrification and diselectrification of the transfer/transport belt 10 are performed by an electrification roller 7 and a diselectrification roller 8, respectively. In addition, for electrification of paper to adsorb it on the belt, an adsorption roller (not shown) is used. By the structure described above, the generation of ozone can be suppressed. The adsorption roller transfers paper from a transport path onto the transfer/transport belt 10 and also fixes it thereon by static adsorption. In addition, after the transfer, for separation of paper from the belt, an adsorption force between paper and the transfer/transport roller 10 is decreased by decreasing a transfer voltage, so that paper can be separated only by means of curvature separation.
As a material of the transfer/transport belt 10, a resistive material and a dielectric material may be used; however, each material has advantages and disadvantages. Since a resistive belt retains charges for a short period of time when being used for transfer operation of the tandem method, charge injection caused by the transfer is low, and even by continuous transfer operation of the four colors, the increase in voltage is relatively small. In addition, even when being used repeatedly for the following paper, the resistive belt releases charges, and hence electrical reset is not required. However, since the electrical resistance of the resistive belt varies with the change in environmental conditions, problems may arise in that the transfer efficiently varies, and/or the thickness and the width of paper adversely influence the transfer performance.
On the other hand, in the case of a dielectric belt, injected changes are not spontaneously released, and hence injection and release of charges must both be electrically controlled. However, since charges are stably retained, adsorption of paper is reliably performed, and hence highly precise paper transport can be performed. In addition, the dielectric constant has a small temperature and humidity dependence, and hence a relatively stable transfer process may be performed in various environments. As disadvantages, the increase in transfer voltage may be mentioned which is caused by accumulation of charges in the belt as the transfer is repeatedly performed.
As a third method, a transfer drum method may be mentioned in which after a recording medium such as paper is wound around a transfer drum, while the drum is allowed to rotate four times, magenta, yellow, cyan, and black toners provided on photosensitive members are sequentially transferred on the medium at respective rotations of the drum, thereby reproducing a color image. According to this method, a relatively high quality image can be obtained; however, when the recording medium is thick such as a postcard, it is difficult to wind the medium around the transfer drum, and hence the type of recording medium is disadvantageously limited.
In addition to the multilayer transfer method, the tandem method, and the transfer drum method described above, as a method in which a high image quality can be obtained, the size of the apparatus is not particularly increased, and the type of recording medium is not particularly limited, an intermediate transfer method has been proposed.
That is, according to this intermediate transfer method, an intermediate transfer member is provided which is composed of a belt and drums designed to temporarily retain toner images transferred from respective four photosensitive members, and four photosensitive members having a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image are disposed around this intermediate transfer member. In the structure described above, the four color toner images are sequentially transferred onto the intermediate transfer member to form a color image thereon, and this color image is then transferred onto a recording medium such as paper. Accordingly, since the gradation is adjusted by superimposing the four toner images, a high image quality can be obtained, and unlike the tandem method, since the photosensitive members are not necessarily aligned, the size of the apparatus is not particularly increased. In addition, the recording medium is not required to be wound around the drum, and hence the type of recording medium is not specifically limited.
As an apparatus forming a color image in accordance with the intermediate transfer method, an image forming apparatus using an endless belt as the intermediate transfer member is shown in FIG. 3 by way of example.
In FIG. 3, reference numeral 11 indicates a drum-shaped photosensitive member which is allowed to rotate in the direction shown by an arrow in the figure. This photosensitive member 11 is electrified by a primary electrifier 12, a part of the member 11 exposed to an image exposure 13 is then diselectrified thereby, an electrostatic latent image corresponding to a first color component is subsequently formed on this photosensitive member 11, the electrostatic latent image is further developed by a developer 41 using a magenta toner M which is the first color, and as a result, the first-color magenta toner image is formed on the photosensitive member 11. Next, this toner image is then transferred onto an intermediate transfer member 20 circularly driven by a drive roller (drive member) 30 while it is being in contact with the photosensitive member 11. In this case, the transfer from the photosensitive member 11 to the intermediate transfer member 20 is performed at a nip portion formed therebetween by a primary transfer bias applied from a power source 61 to the intermediate transfer member 20. After the first-color magenta toner image is transferred onto this intermediate transfer member 20, the surface of the photosensitive member 11 is cleaned by a cleaning device 14, and hence a first development and transfer operation of the photosensitive member 11 is complete. Subsequently, while the photosensitive member 11 is allowed to rotate three times, at the respective rotations, a second-color cyan toner image, a third-color yellow toner image, and a fourth-color black toner image are sequentially formed in that order on the photosensitive member 11 at the respective rotations by sequentially using developers 42 to 44 so that the four color images are superimposed on the intermediate transfer member 20 at the respective rotations, and hence a composite color toner image corresponding to an object color image is formed on the intermediate transfer member 20. In the apparatus shown in FIG. 3, at the respective rotations of the photosensitive member 11, the positions of the developers 41 to 44 are changed so that development of magenta toner M, cyan toner C, yellow toner Y, and black toner B are sequentially performed.
Next, a transfer roller 25 is then brought into contact with the intermediate transfer member 20 provided with the composite color toner image thereon, and to a nip portion therebetween, a recording medium 26 is supplied from a paper feed cassette 19. At the same time, a secondary transfer bias is applied to the transfer roller 25 from a power source 29, and the composite color toner image is transferred from the intermediate transfer member 20 onto the recording medium 26, followed by heating and fixing, thereby forming a final image. After the composite color toner image is transferred onto the recording medium 26, the intermediate transfer member 20 is processed by a cleaning device 35 so as to remove residual toners remaining on the surface and is then placed in a standby state for the following image formation.
In addition, a tandem type intermediate transfer method formed in combination between the tandem method and the intermediate transfer method has also been proposed. In FIG. 4, an image forming apparatus in accordance with a tandem type intermediate transfer method is shown by way of example in which color image formation is performed using an endless belt-shaped tandem type intermediate transfer member.
In the apparatus shown in the figure, a first development portion 54a to a fourth development portion 54d are sequentially disposed along a tandem type intermediate transfer member 50 for developing electrostatic latent images on photosensitive drums 52a to 52d using yellow, magenta, cyan, and black toners, respectively, and this tandem type intermediate transfer member 50 is circularly driven in the direction indicated by an arrow shown in the figure, so that four color toner images formed on the photosensitive drums 52a to 52d of the respective development portions 54a to 54d are sequentially transferred on this tandem type intermediate transfer member 50 to form a color toner image thereon. Subsequently, this toner image thus formed is transferred onto a recording medium 53 such as paper by transfer, thereby performing printout.
In the figure, reference numeral 55 indicates a drive roller or a tension roller circularly driving the tandem type intermediate transfer member 50, reference numeral 56 indicates a recording medium feed roller, reference numeral 57 indicates a recording medium feed device, and reference numeral 58 indicates a fixing device fixing an image on the recording medium by heating or the like. In addition, reference numeral 59 indicates a power source device (voltage application means) applying a voltage to the tandem type intermediate transfer member 50, and this power source device 59 is designed to be able to change the application direction of the voltage depending on the case in which the toner image is transferred onto the tandem type intermediate transfer member 50 from the photosensitive drums 52a to 52d and the case in which the toner image is transferred from the tandem type intermediate transfer member 50 to the recording medium 53.
In the various image forming apparatuses described above, as a conductive endless belt used as the transfer/transport belt 10, the intermediate transfer member 20, the tandem type intermediate transfer member 50, and the like, heretofore, a semiconductive resin film belt and a fiber reinforced rubber belt have been primarily used. Of the belts described above, as the semiconductive resin film belt, for example, a belt composed of a polycarbonate resin and carbon black compounded therewith, a belt primarily composed of a poly(alkylene terephthalate) resin, and a belt primarily composed of a thermoplastic polyamide resin have been proposed.
In addition, for example, in Japanese Unexamined Patent Application Publication No. 2001-125396, in order to improve the transfer efficiency and the image quality, a transfer belt has been disclosed having the structure composed of a tube member formed of a single layer or a laminate using a polymer material, a conductive layer provided on the outside surface of the tube member, and a semiconductive resin layer of a material having semiconductive properties provided on the above conductive layer. Furthermore, in Japanese Unexamined Patent Application Publication No. 2002-365866, in order to realize an electrophotographic seamless belt having uniform electrical resistance and superior surface and mechanical properties, a technique has been disclosed in which predetermined amounts of poly(ether ester amide) or poly(ether amide) and a fluorinated surfactant or a halogenated alkali metal are contained in a belt composed of a thermoplastic fluorinated resin.
As described above, as for the conductive endless belt, various researches have been conducted; however, concomitant with recent demanding requirements of performance, a belt having superior various properties has been increasingly desired. In particular, a conductive endless belt has been desired having small voltage dependence of resistance as electrical properties, superior durability in image output, and high toner transfer efficiency.
In addition, in the resin film belt described above, in order to obtain desired electrification properties required for the member, the adjustment of the conductivity has been generally performed by adding conductive materials such as an electron conductive agent or an ionic conductive agent; however, in electron-conduction belts using an electron conductive agent such as conductive carbon including carbon black filler, the voltage dependence of electrical resistance is large when a voltage is applied, and in particular, when a high voltage is applied, problems such as leakage may occur in some cases. Furthermore, when a low molecular weight-based antistatic agent is used, for example, various problems occur: the antistatic properties are degraded with time when electricity is supplied, and due to bleedout of a low molecular weight-based material, other constituent elements in contact therewith are contaminated.